Natural cell-mediated cytotoxicity is the function of at least two major populations, natural killer (NK) and natural cytotoxic (NC) cells. We have characterized NC cells with regard to their regulation by lymphokines, their target cell specificity and cell surface phenotype. The studies proposed in this application are designed to further characterize the NC cell system with regard to its mechanism of killing and target recognition structures. Using cloned thymus leukemia cell lines with and without NC cell activity, we will 1) characterize morphologically and serologically the NC+ leukemia clones in order to determine characteristics of the NC phenotype; 2) examine the mechanism of NC lysis at the single cell level and its regulation; and 3) by generating clonotypic anti-NC monoclonal antibodies, characterize the NC target recognition structures. The use of cloned NC+ and NC- thymus leukemias should provide a model system for analyzing the NC system and, by providing an ideal screening system, allow for the generation of monoclonal antibodies recognizing NC associated cell surface antigens and receptors. In addition, studies are planned which will further analyze the role of NC activity in vivo. Utilizing both NC susceptible and resistant tumors, studies are planned which will correlate susceptibility to NC in vitro with tumorigenicity in vivo. Similarly, the role of NC in surveillance against metastasis will be examined by the iv injection of NC susceptible and resistant tumor lines and the subsequent quantitation of tumor foci in the lung. The utilization of these two methods should allow for the characterization of the in vivo function of NC.